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5 X-ray Pulsar-Based Navigation: Theories and Experiments
5.3.2 Concepts and Classifications of Time
The time contains two basic concepts, the moment and the time interval. The socalled moment refers to the time when a phenomenon or an event occurs, and
the so-called time interval refers to the process experienced by a phenomenon or
an event, which is numerically equal to the difference between the start and end
moments of the phenomenon or event. In astronomy and navigation application, the
moment of recording an event is called the epoch; the unit used to measure or meter
the time interval is called timescale; the starting point of measuring the moment is
called time origin or initial epoch. The time system is a time measurement reference
standard composed of the time origin and timescale, also known as time reference.
Generally, the timescale is the core of establishing the time reference, and the time
origin can be chosen according to the practical application.
If a continuous changing physical quantity x is expressed as a function taking the
time t as an independent variable, and by measuring the change of the x, the change
characteristics of the physical quantity are recorded accurately, then the x can be used
as a kind of means of measuring time. Usually, it is expected that the time function
of the selected physical quantity is a simple and linear function form, namely,
x = f (t) = a + bt,
(5.7)
where a is the time origin; when t = 0, x = a; b is the timescale.
Generally, formula (5.7) is called time measurement equation. From the practice
of establishing the time system, it is shown that it is most convenient to choose the
physical quantity with observable periodic motion as the time reference. In this way,
after a certain time interval or motion period, all states of the physical quantity can
be reproduced, which is advantage to the accurate measurement of the moment and
time interval. Therefore, the basic conditions for selecting an observable physical
quantity as a time reference are as follows:
(1) The motion should be continuous and periodic.
(2) The period of motion should have enough stability, and at any time, the period
of the motion should be constant, seldom affected by the change of external
environment conditions.
(3) The periodic phenomenon of the motion should be reproducible. That is to say,
the periodic motion can be reproduced by the observations and experiments at
any time and any place.
Due to the difference of the selected periodic motion phenomena, there are
different time systems. Actually, there is not an ideal time reference that fully
meets the above basic conditions in the objective world. Generally, it can only be
approximately consistent, and thus the actual time measurement equation should be
expressed as
x = f (t) = a + bt + φ(t),
(5.8)
5 X-ray Pulsar-Based Navigation: Theories and Experiments
5.3.2 Concepts and Classifications of Time
The time contains two basic concepts, the moment and the time interval. The socalled moment refers to the time when a phenomenon or an event occurs, and
the so-called time interval refers to the process experienced by a phenomenon or
an event, which is numerically equal to the difference between the start and end
moments of the phenomenon or event. In astronomy and navigation application, the
moment of recording an event is called the epoch; the unit used to measure or meter
the time interval is called timescale; the starting point of measuring the moment is
called time origin or initial epoch. The time system is a time measurement reference
standard composed of the time origin and timescale, also known as time reference.
Generally, the timescale is the core of establishing the time reference, and the time
origin can be chosen according to the practical application.
If a continuous changing physical quantity x is expressed as a function taking the
time t as an independent variable, and by measuring the change of the x, the change
characteristics of the physical quantity are recorded accurately, then the x can be used
as a kind of means of measuring time. Usually, it is expected that the time function
of the selected physical quantity is a simple and linear function form, namely,
x = f (t) = a + bt,
(5.7)
where a is the time origin; when t = 0, x = a; b is the timescale.
Generally, formula (5.7) is called time measurement equation. From the practice
of establishing the time system, it is shown that it is most convenient to choose the
physical quantity with observable periodic motion as the time reference. In this way,
after a certain time interval or motion period, all states of the physical quantity can
be reproduced, which is advantage to the accurate measurement of the moment and
time interval. Therefore, the basic conditions for selecting an observable physical
quantity as a time reference are as follows:
(1) The motion should be continuous and periodic.
(2) The period of motion should have enough stability, and at any time, the period
of the motion should be constant, seldom affected by the change of external
environment conditions.
(3) The periodic phenomenon of the motion should be reproducible. That is to say,
the periodic motion can be reproduced by the observations and experiments at
any time and any place.
Due to the difference of the selected periodic motion phenomena, there are
different time systems. Actually, there is not an ideal time reference that fully
meets the above basic conditions in the objective world. Generally, it can only be
approximately consistent, and thus the actual time measurement equation should be
expressed as
x = f (t) = a + bt + φ(t),
(5.8)